In agricultural production, a number of residues is originated in the elaboration of agricultural products, which are occasionally further treated, e.g. wood chips to produce cellulose, corn straw as litter or corn stalks for silage. In case the supply of the above materials exceeds their demand, it is possible to use some of them, e.g. wood chips, for the generation of energy by their combustion, nevertheless, other materials, e.g. corn straw, cannot be simply burnt down and if grate boilers of special design are not used, e.g. boilers with water-cooled grates in which combustion gases are cooled down in the combustion chamber, it is necessary to apply the inefficient deposition.5 
Most materials considered as fuels have a high ash-melting temperature, usually of more than 1,300° C. Other materials containing carbon and thus potentially combustible materials, such as corn straw, pressed work from the production of methyl ester, or residues from the production of sunflower-seed oil, have a lower ash-melting temperature, which is usually in the range of 850-1,050° C. Specifically, the ash-melting temperature of corn straw is about 850° C., of corn and meals from corn 620 to 750° C. and even less, of residues from sunflower-seed production 1,010° C., pressed work from rape-seed methyl esters about 1,210° C. The low ash-melting temperature causes build-up of slag in the combustion chamber, in the boiler combustion space and even on the heat transfer surfaces and thus it challenges the operation of the boiler and sometimes even makes its operation impossible. In fluidized bed boilers, these materials are not burnt due to the low temperature of ash matter.
At present, due to the efforts to get rid of the dependence on in the import of fossil fuels, in particular crude oil, the interest in the use of hitherto unused organic matter, especially waste organic matter, for the production of energy, has been growing. The interest is especially focused on the production bioethanol, which could be produced in every country from its local agricultural crops and thus crude oil could be replaced for the greater part with it. For the production bioethanol, sugary and starch crops can be used, in particular sugar beet and cereals. Produced bioethanol can be used directly in the combustion engines as fuel, nevertheless, at present it is usually used as an admixture, which is added in the amounts of 5% to 10% to the conventional mineral fuels. Using ethanol, the octane number is increased and the amount of CO2 emissions decreased.
The sugarcane ethanol has found its wide application especially in Brazil, where it is used as automotive fuel. In the 80-ies of the last century, approximately two-thirds of cars in Brazil had their engines specially adapted for the combustion of pure bioethanol. At present, the new cars are not adapted in this way any more, nevertheless, all motor-vehicle petrol in Brazil contains 26% of sugarcane bioethanol. Current combustion engines can run on this mixture.
The bioethanol produced from maize is also used as a fuel additive to most automotive petrols in the USA, usually as a 10% admixture.
The waste in the production of bioethanol from maize, cereals or sugarcane, sugar beet is stillage. The stillage could also be potentially used as fuel, nevertheless, due to its low ash-melting temperature, it is not used as fuel. At present, the stillage is usually centrifuged on a centrifuge, the solid fraction, called cake, is dried in a steam drier or a drum drier, where the drier heating medium is fossil fuel. The liquid fraction, called centrate, is condensed in multi-stage evaporators to obtain syrup, which is added to the solid fraction, the cake, and it is dried together with it in a steam drier or a drum drier. Thus a by-product is produced, which is used as a forage mixture in the form of dried granules. The unused part is also dumped into the sea in the countries having appropriate conditions for it.
The major part of stillage, especially due to the ever growing production of bio-ethanol, is thus not used any more, it goes beyond its use as forage, and thus it is indispensable to look for other possibilities of its application. Its direct combustion is thus impossible, as stillage without drying has low caloric power and it usually contains approx. 90% of water. If stillage is dried to 40-30-10% of moisture content, its caloric power would increase to approx. 9-13-17 MJ/kg. From the aspect of energy, such dried stillage is thermally self-supporting, which lays down one of the prerequisites for its combustion in current boilers. Nevertheless, it has been found that dried stilled can not be combusted in current boilers, at least nobody has been successful in it so far due to its low-temperature melting ashes. When combusting this sort of stillage, the low melting temperature of ashes both in grate boilers and the fluidized bed boilers and dry bottom boilers results in the fact that melted ash agglomerates the fluidized bed, it forms slag in the furnace and on the heat exchange surfaces of the boiler, resulting in a breakdown of the boiler, which is put out of service in a matter of a few minutes.
The experts have been hitherto of the opinion that the problem of exploiting biowaste with low ash-melting temperature could be solved by adapting the boiler. Nevertheless, up to now nobody has succeeded in it.
Another possible method of exploiting stillage is the system in which the liquid stillage as a whole is conveyed to the biogas station to produce biogas from it. The disadvantage of this method is the fact that biogas is produced only from the liquid fraction of the stillage and thus the solid fraction is practically wasted. Prolongation of the decomposition time would increase the biogas yield, nevertheless, it would be at the price of higher investment and operation costs. In addition to it, the solid residue after the utilization of stillage in the biogas station would be higher by 50%, even in case of a very long fermentation period. This system thus does not find any solution to the problem of the stillage solid fraction.